1. Field of the Invention
The present invention relates to matrix printing methods and in particular to a method for reducing the redundancy of binary character sequences employed for matrix printing.
2. Description of the Prior Art
In the punctiform representation of alphanumeric characters and/or graphics patterns by means of conventional matrix printing methods, the characters or patterns are represented by a selected plurality of individual points in the raster of a character matrix. Mosaic printing heads or ink printing heads are employed as the printing means for the matrix printing methods. In one type of printing head, the printing elements are needles which are struck against a recording medium after actuation by suitable drive elements such as, for example, plunger type armature magnet systems. The impression of the printing needles causes the printing of individual points on the recording medium.
In another mosaic printing method, individual ink droplets are sprayed against the recording medium from jets of a mosaic printing head which are driven by piezo-ceramic drive elements thus forming the points of the character to be represented on the recording media. The printing quality of the characters generated by either of the above two means is substantially dependent upon the resolution of the points comprising the particular character or pattern. The resolution is determined by the number of individual points per character, that is, by the number of rows and columns in the character matrix. In mosaic printing methods, the number of printing needles or the number of ink jets determines the number of rows in the character raster.
The columns of the character matrix or raster are determined by the relative motion between the printing head and the recording medium, which requires a selected fixed spacing between the columns. A printing element can be actuated again after a first actuation only after the passage of a specific time span, which is referred to in the art as a step or as a printing step. When the mosaic printing head provided with the printing elements is moved by a whole printing step between two actuations of printing elements, a low resolution is achieved. Movement of the printing elements of a mosaic printing head by partial step lengths between two actuations of the printing elements in order to improve the character resolution is described, for example, in German LP No. 27 48 289.
The information for driving the individual pointgenerating printing elements for a particular character to be printed is received by a printing control unit from a character generator in the form of binary character sequences which are converted into drive pulses for the printing elements. An increase in the resolution of the printed character which, as stated above, is achieved by means of a greater number of raster points within the character raster, requires a considerable character capacity for the character generator and results in relatively long processing times. For example, a storage capacity of 731 Kbit is required for a character raster which exhibits 28 rows and 102 columns, and therefore contains 2856 image points. Such a raster permits the printing of 256 different characters. Given a printing speed of 200 characters per second, a character throughput of 81.6 Kbytes per second results, meaning a processing time of 12.25 .mu.sec per byte. Conventional microprocessors are not capable for processing such data volumes at this rate.